The present invention relates generally to intraocular lens (IOL) assemblies and particularly to accommodating IOL assemblies.
Natural accommodation in a normal human eye having a normal human crystalline lens involves automatic contraction or constriction and relaxation of the ciliary muscle of the eye (and zonules controlled by the ciliary muscle) by the brain in response to looking at objects at different distances. Ciliary muscle relaxation, which is the normal state of the muscle, shapes the human crystalline lens for distant vision. Ciliary muscle contraction shapes the human crystalline lens for near vision. The brain-induced change from distant vision to near vision is referred to as accommodation.
Accommodating intraocular lens (IOL) assemblies have been developed that comprise an IOL that moves in response to ciliary muscular contraction and relaxation, thereby to simulate the movement of the natural lens in the eye, and, inter alia, help provide patients with better focusing ability.
One example of an accommodating IOL assembly is described in U.S. Pat. No. 5,476,514 to Cumming, the disclosure of which is incorporated herein by reference. Cumming describes an accommodating IOL with plate haptics for implantation within the capsular bag of the eye, after removal of the natural lens by an anterior capsulotomy. During a postoperative healing period following surgery, anterior capsular remnants fuse to the posterior capsule of the bag by fibrosis about the plate haptics, and the lens is deflected rearward to a distant vision position against the elastic posterior capsule of the bag in which the posterior capsule is stretched rearward. After fibrosis is complete, natural brain-induced contraction and relaxation of the ciliary muscle relaxes and stretches the fused remnants and increases and reduces vitreous pressure in the eye to effect vision accommodation by the fused remnants, the posterior capsule, and vitreous pressure.
Haptics are clearly defined in the art as the interface elements of the IOL that touch the eye structure (xe2x80x9chapticxe2x80x9d is from the Greek word for touch). It is noted that in Cumming, movement, contraction or tensioning of the haptics causes the lens movement. This is a drawback because the mechanical behavior of the haptics is affected by the boundary conditions between the haptics and the eye structure. These boundary conditions are not well defined and may not be constant, since they arise from the amount and nature of the fibrosis about the plate haptics.
Another example of an accommodating IOL assembly is described in U.S. Pat. No. 6,013,101 to Israel, the disclosure of which is incorporated herein by reference. The IOL assembly includes at least two, preferably rigid, linkage arms, i.e., haptics, each being attached to the optic at a first position on the arm thereof and cooperating with ciliary muscle or the zonules at a second position on the arm. There are at least two pivots, one of which is rotatably attached to each respective haptic intermediate the first and second positions.
U.S. Pat. No. 6,013,101 discusses the difference between what it calls xe2x80x9crigidxe2x80x9d haptics or linkage arms and xe2x80x9cflexiblexe2x80x9d or xe2x80x9cresilientxe2x80x9d haptics. Resilient haptics comprise resilient wires formed of plastics or any other biologically inert material, which are sufficiently stiff so that when a compressive force is applied thereto, they distort but do not buckle or collapse. When compressed, resilient haptics cause the artificial lens to translate anteriorly along the optical axis (anterior-posterior axis). When the compressive force is reduced, the resilient haptics spring back under their own elasticity so as to return the lens to its original position.
Rigid haptics, on the other hand, do not deform significantly under the compressive or tensile forces present during accommodation. They are, therefore, capable of transmitting forces applied to them more efficiently than flexible elements and potentially with greater mechanical advantage. It is in this context that the term xe2x80x9crigidxe2x80x9d is to be understood throughout the disclosure. It should be understood, however, that xe2x80x9crigidxe2x80x9d haptics may be made of very thin material and may not be rigid under other circumstances, such as during surgical implantation, when greater force is applied to them so that they can be inserted into the lens capsule.
However, the mechanical behavior of the haptics of U.S. Pat. No. 6,013,101 is also affected by the boundary conditions between the haptics and the eye structure.
The present invention seeks to provide an improved accommodating IOL assembly. In contrast to the prior art, in the present invention, the haptic does not apply leverage to the artificial lens. Rather one or more leverage arms, which connect the haptic to the lens, apply the requisite lever force to the lens to impart accommodating motion to the lens. In one embodiment of the invention, the leverage arm applies a lever force along a chord inwards of the perimeter of the lens. In other words, the leverage arm has a significantly greater xe2x80x9creachxe2x80x9d and mechanical advantage than prior art accommodating IOLs, which rely on circumferentially attached haptics to apply leverage to the lens.
Thus, in the present invention, accommodating movement of the lens is accomplished by applying the lever force from the leverage arm, which force is generally independent of the boundary condition of the haptic. The haptic of the present invention is preferably a ring haptic.
There is thus provided in accordance with a preferred embodiment of the present invention an intraocular lens assembly including a lens, a haptic, and a leverage arm connecting the lens to the haptic, wherein the leverage arm is adapted to apply a lever force on the lens acting generally along a chord inwards of a perimeter of the lens.
In accordance with a preferred embodiment of the present invention the leverage arm has a longitudinal axis, and a first end attached to the haptic and a second end attached to the perimeter of the lens offset from the longitudinal axis.
Further in accordance with a preferred embodiment of the present invention the first end of the leverage arm is generally symmetric about the longitudinal axis.
Still further in accordance with a preferred embodiment of the present invention the second end of the leverage arm includes two attachment zones generally symmetric about the longitudinal axis. The chord may connect the two attachment zones and intersect the longitudinal axis between the first end and a center of the lens.
In accordance with a preferred embodiment of the present invention the two attachment zones have a span of at least 90xc2x0 therebetween with respect to a center of the lens.
Further in accordance with a preferred embodiment of the present invention the leverage arm is configured generally as a plate. The haptic may be shaped generally like a ring.
There is also provided in accordance with a preferred embodiment of the present invention a method for causing movement of a lens of an intraocular lens assembly, the method including attaching a lens to a haptic with a leverage arm, and causing movement of the lens by applying a lever force from the leverage arm on the lens generally independent of a boundary condition of the haptic. The movement of the lens is preferably generally along an anterior-posterior ocular axis.